Optical ignition of fuels

ABSTRACT

A method of combustion includes: (1) introducing microparticles and nanoparticles into a combustion chamber, where the microparticles and the nanoparticles are formed of different materials; and (2) using an optical source, irradiating the microparticles and the nanoparticles within the combustion chamber to ignite the microparticles.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser. No. 61/773,953 filed on Mar. 7, 2013, the disclosure of which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No. W911NF-10-1-0106, awarded by the Army Research Office. The Government has certain rights in the invention.

FIELD OF THE INVENTION

This disclosure generally relates to ignition of fuels and, more particularly, to optical ignition of fuels.

BACKGROUND

Combustion of fuels provides a driving force for a number of applications, including stationary power generation and transportation. Fuels are typically ignited by spark plugs, hot wires, and pilot flames, where chemical reactions are initiated locally and propagate to the rest of a fuel volume. Since ignition occurs at a single location, incomplete combustion can occur when there is insufficient time for reaction, such as in rockets and scramjets. In addition, combustion of fuels is desirable to generate power in microelectromechanical systems (MEMS), but the limited space available impedes the use of conventional ignition systems.

It is against this background that a need arose to develop the technique for optical ignition of fuels and related systems and methods described herein

SUMMARY

Distributed, non-intrusive, and miniaturizable ignition systems are desired for controlling combustion, and for allowing integration into MEMS as power generators. Embodiments described herein are directed to a distributed, optical ignition technique that uses an optical source to ignite particles of energetic materials, resulting in the ignition of solid phase energetic materials, liquid fuels, and gaseous fuels. In some embodiments, the optical ignition occurs when the particles have suitable dimensions to cause a temperature rise above their ignition temperatures.

For example, aluminum (Al) is an attractive solid fuel for rocket propulsion and energy conversion systems due to its large volumetric energy density, earth abundance, and low cost. Non-intrusive optical flash ignition is attractive for many applications due to its simplicity and flexibility in controlling the area exposed to the flash. However, flash ignition of Al microparticles can be challenging due to their higher minimum flash ignition energy, which may originate from weaker light absorption and higher ignition temperature compared to Al nanoparticles. Herein for some embodiments, the minimum flash ignition energy of Al microparticles is reduced by the addition of metal oxide nanoparticles.

Some aspects of this disclosure are directed to a method of combustion. In some embodiments, the method includes: (1) introducing microparticles and nanoparticles into a combustion chamber, where the microparticles and the nanoparticles are formed of different materials; and (2) using an optical source, irradiating the microparticles and the nanoparticles within the combustion chamber to ignite the microparticles.

Other aspects of this disclosure are directed to a reaction device. In some embodiments, the reaction device includes: (1) a housing defining an internal chamber; (2) a reaction material disposed within the internal chamber and including microparticles and nanoparticles that are formed of different materials; and (3) an optical ignition system connected to the housing and operable to irradiate the microparticles and the nanoparticles to ignite the reaction material.

Other aspects and embodiments of this disclosure are also contemplated. The foregoing summary and the following detailed description are not meant to restrict this disclosure to any particular embodiment but are merely meant to describe some embodiments of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of some embodiments of this disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.

FIG. 1: A reciprocating engine implemented in accordance with an embodiment of this disclosure.

FIG. 2: A jet engine implemented in accordance with another embodiment of this disclosure.

FIG. 3: A reaction device implemented in accordance with yet another embodiment of this disclosure.

FIG. 4: Flash ignition of Al microparticles (MPs) with addition of WO₃ nanoparticles (NPs). (a) Schematic of an experimental setup for ignition of a mixture of Al MPs (about 2.3 μm) and WO₃ NPs (about 80 nm) by a xenon flash. (b) Optical images of a mixture of Al MPs and WO₃ NPs (φ=1, φ_(n)=0.5) before and after the xenon flash exposure.

FIG. 5: Comparison of Scanning Electron Microscopy (SEM) images of a mixture Al MPs (about 2.3 μm) and WO₃ NPs (about 80 nm, φ=1, φ_(n)=0.5). (a) Before and (b) after the xenon flash exposure. Insets: enlarged view of the corresponding product surface. (c) X-ray diffraction pattern of the mixture of Al MPs and WO₃ NPs after ignition.

FIG. 6: Minimum flash ignition energy of Al MPs with addition of WO₃ NPs. (a) Large Al MPs (about 2.3 μm) and (b) small Al MPs (about 0.9 μm) mixed with WO₃ NPs with respect to normalized equivalence ratios in air and nitrogen gas.

FIG. 7: Optical characterizations of Al MPs with addition of WO₃ NPs. Absorption plus scattering of (a) large Al MPs (about 2.3 μm) and (b) small Al MPs (about 0.9 μm) mixed with WO₃ NPs with respect to normalized equivalence ratios over the wavelength of 300-1100 nm.

DETAILED DESCRIPTION Definitions

The following definitions apply to some of the aspects described with respect to some embodiments of this disclosure. These definitions may likewise be expanded upon herein.

As used herein, the singular terms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an object can include multiple objects unless the context clearly dictates otherwise.

As used herein, the term “set” refers to a collection of one or more objects. Thus, for example, a set of objects can include a single object or multiple objects. Objects of a set can also be referred to as members of the set. Objects of a set can be the same or different. In some instances, objects of a set can share one or more common characteristics.

As used herein, the terms “connect,” “connected,” “connecting,” and “connection” refer to an operational coupling or linking Connected objects can be directly coupled to one another or can be indirectly coupled to one another, such as through another set of objects.

As used herein, the terms “substantially” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. For example, the terms can refer to less than or equal to ±10%, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

As used herein, the term “aspect ratio” refers to a ratio of a largest dimension or extent of an object and an average of remaining dimensions or extents of the object, where the remaining dimensions are orthogonal with respect to one another and with respect to the largest dimension. In some instances, remaining dimensions of an object can be substantially the same, and an average of the remaining dimensions can substantially correspond to either of the remaining dimensions. For example, an aspect ratio of a cylinder refers to a ratio of a length of the cylinder and a cross-sectional diameter of the cylinder. As another example, an aspect ratio of a spheroid refers to a ratio of a major axis of the spheroid and a minor axis of the spheroid.

As used herein, the term “size” refers to a characteristic dimension of an object. Thus, for example, a size of an object that is spherical can refer to a diameter of the object. In the case of an object that is non-spherical, a size of the non-spherical object can refer to a diameter of a corresponding spherical object, where the corresponding spherical object exhibits or has a particular set of derivable or measurable characteristics that are substantially the same as those of the non-spherical object. Thus, for example, a size of a non-spherical object can refer to a diameter of a corresponding spherical object that exhibits optical characteristics that are substantially the same as those of the non-spherical object. Alternatively, or in conjunction, a size of a non-spherical object can refer to an average of various orthogonal dimensions of the object. Thus, for example, a size of an object that is a spheroidal can refer to an average of a major axis and a minor axis of the object. When referring to a set of objects as having a particular size, it is contemplated that the objects can have a distribution of sizes around the particular size. Thus, as used herein, a size of a set of objects can refer to a typical size of a distribution of sizes, such as an average size, a median size, or a peak size.

As used herein, the term “nanostructure” refers to an object that has at least one dimension in the range of about 1 nm to about 100 nm, such as from about 1 nm to about 50 nm, from about 50 nm to about 100 nm, from about 1 nm to about 20 nm, from about 20 nm to about 100 nm, from about 1 nm to about 10 nm, or from about 10 nm to about 100 nm. A nanostructure can have any of a wide variety of shapes, and can be formed of a wide variety of materials.

As used herein, the term “nanoparticle” refers to a spherical or spheroidal nanostructure. Typically, each dimension of a nanoparticle is in the range of about 1 nm to about 100 nm, the nanoparticle has a size in the range of about 1 nm to about 100 nm, and the nanoparticle also has an aspect ratio that is less than about 5, such as no greater than about 3, no greater than about 2, no greater than about 1.5, or about 1.

As used herein, the term “microstructure” refers to an object that has at least one dimension in the range of about 100 nm to about 100 μm, such as from about 100 nm to about 500 nm, from about 500 nm to about 1 μm, from about 1 μm to about 20 μm, from about 20 μm to about 100 μm, from about 1 μm to about 5 μm, from about 5 μm to about 10 pm, from about 1 μm to about 10 μm, or from about 10 μm to about 100 μm. A microstructure can have any of a wide variety of shapes, and can be formed of a wide variety of materials.

As used herein, the term “microparticle” refers to a spherical or spheroidal microstructure. Typically, each dimension of a microparticle is in the range of about 100 nm to about 100 μm, the microparticle has a size in the range of about 100 nm to about 100 μm, and the microparticle also has an aspect ratio that is less than about 5, such as no greater than about 3, no greater than about 2, no greater than about 1.5, or about 1.

As used herein, the term “ultraviolet range” refers to a range of wavelengths from about 5 nm to about 400 nm.

As used herein, the term “visible range” refers to a range of wavelengths from about 400 nm to about 700 nm.

As used herein, the term “infrared range” refers to a range of wavelengths from about 700 nm to about 2 mm.

Optical Ignition of Fuels

Embodiments described herein provide a technique to ignite solid, liquid, and gaseous fuels in a distributed, multi-point, or homogenous fashion for combustion applications. It is desirable to have distributed ignition to reduce the time for combustion and achieve higher combustion efficiency through greater uniformity of reaction. Shorter combustion time and higher combustion efficiency are desirable for a number of applications, including rapid reciprocating engines and jet engines.

According to some embodiments, a fuel and a set of particles (either, or both, nanoparticles and microparticles) are injected separately or as a mixture into a combustion chamber, and the particles are exposed to optical energy, such as a short pulse of light. The distribution of the particles within a volume of the fuel allows chemical reactions to be initiated at multiple locations. Specifically, the particles absorb the optical energy and release it as heat, and the released heat is sufficiently high to ignite the fuel in a distributed fashion. In other embodiments, the particles can serve as the fuel itself, such that an additional fuel can be omitted.

According to some embodiments, the use of either, or both, nanoparticles and microparticles provides various advantages. Certain of these advantages derive from a higher surface to volume ratio, with the higher surface to volume ratio providing improved performance in terms of absorption of optical energy and its conversion into heat. Also, the use of either, or both, nanoparticles and microparticles allows ignition to occur without requiring an additional oxidant beyond air. Moreover, ignition can occur without relying on the presence of embedded catalysts that can translate into higher manufacturing and operational costs.

According to some embodiments, microparticles are formed of, or include, energetic materials having a high energy density that can be released as heat for ignition, such as an energy density of at least about 20 kJ/cm³, at least about 30 kJ/cm³, at least about 40 kJ/cm³, at least about 50 kJ/cm³, at least about 60 kJ/cm³, at least about 70 kJ/cm³, or at least about 80 kJ/cm³, and up to about 100 kJ/cm³, up to about 150 kJ/cm³, up to about 200 kJ/cm³, or more. Examples of such energetic materials include metals, such as alkali metals (e.g., lithium, sodium, potassium, rubidium, and cesium), alkaline earth metals (e.g., magnesium), transition metals (e.g., iron (Fe), copper (Cu), tungsten (W), and molybdenum (Mo)), post-transition metals (e.g., aluminum (Al)), lanthanides, and actinides; and alloys, oxides, hydrides, and alkoxides of such metals, such as aluminum oxide (e.g., Al₂O₃), iron oxide (e.g., Fe₂O₃), copper oxide (e.g., CuO), tungsten oxide (e.g., W0₃), and molybdenum oxide (e.g., MoO₃). Additional examples of such energetic materials include pyrophoric materials. According to some embodiments, microparticles each include a core that is formed of, or include, an energetic material, which core is partially or fully surrounded by a shell that is formed of, or includes, the same energetic material or a different material. A thickness of the shell can be up to about 20 nm, such as up to about 15 nm, up to about 10 nm, or up to about 5 nm, and down to about 1 nm or less. In some embodiments, the core is formed of, or includes, a metal, and the shell is formed of, or includes, an oxide of the same metal or a different metal. The shell can provide a protective function, as well as serve as an oxidant during ignition.

Compared to nanoparticles, microparticles of an energetic material can be more suitable for some embodiments since they are cheaper, safer to handle, and include a higher content of the energetic material due to a smaller fraction of an inert shell. However, it can be difficult to optically ignite microparticles due to their low light absorption and high ignition temperature.

According to some embodiments, a mixture of microparticles and nanoparticles formed of, or including, different materials can be used. Specifically, the mixture can include microparticles formed of, or including, a first energetic material, along with nanoparticles formed of, or including, a second material that promotes ignition of the first energetic material, such as by providing a source of oxygen, enhancing light absorption, or both. Examples of such a mixture include a mixture of Al microparticles and nanoparticles of a metal oxide, such as WO₃ nanoparticles, Fe₂O₃ nanoparticles, MoO₃ nanoparticles, or any combination thereof. In some embodiments, a molar ratio of the second material to the first energetic material is in the range of about 1:10 to about 10:1, such as from about 1:10 to about 5:1, from about 5:1 to about 10:1, from about 1:10 to about 3:1, from about 3:1 to about 10:1, from about 1:10 to about 2:1, from about 2:1 to about 10:1, from about 1:10 to about 1:1, from about 1:1 to about 10:1, from about 1:7 to about 1:1, from about 1:5 to about 1:1, from about 1:3 to about 1:1, from about 1:10 to about 2:3, from about 1:7 to about 2:3, from about 1:5 to about 2:3, or from about 1:3 to about 2:3. In some embodiments, the second material has a bandgap energy that falls within an emission spectrum of an optical source used for flash ignition, such as within the ultraviolet range, the visible range, or the infrared range.

According to some embodiments, a mixture of microparticles and nanoparticles are ignited by an optical source through a photo-thermal effect. Suitable optical sources include pulsed sources that provide incident light with an energy density in the range of about 0.1 J/cm² to about 100 J/cm², such as from about 0.1 J/cm² to about 50 J/cm², from about 0.1 J/cm² to about 10 J/cm², from about 0.1 J/cm² to about 5 J/cm², or from about 0.1 J/cm² to about 1 J/cm², and a pulse duration in the range of about 0.1 ms to about 100 ms, such as from about 0.1 ms to about 50 ms or from about 0.1 ms to about 10 ms. Advantageously, the inclusion of the nanoparticles, such as nanoparticles of a metal oxide, can reduce a threshold or minimum energy density that can ignite the microparticles, such as no greater than about 1 J/cm², no greater than about 0.95 J/cm², no greater than about 0.9 J/cm², no greater than about 0.85 J/cm², no greater than about 0.8 J/cm², no greater than about 0.75 J/cm², no greater than about 0.7 J/cm², no greater than about 0.65 J/cm², no greater than about 0.6 J/cm², no greater than about 0.55 J/cm², no greater than about 0.5 J/cm², no greater than about 0.45 J/cm², no greater than about 0.4 J/cm², or no greater than about 0.35 J/cm². Through the photo-thermal effect, irradiation of the mixture by an optical source can yield a heating rate in the range of about 10⁵ K/s to about 10⁹ K/s, such as from about 10⁵ K/s to about 10⁸ K/s, from about 10⁶ K/s to about 10⁸ K/s, or from about 10⁵ K/s to about 10⁷ K/s. For example, an electronic flashtube, such as a xenon lamp of a camera flash unit, can be a suitable optical source. Flash ignition has the advantages of low power input, multi-point initiation, and broad emission spectrum across the ultraviolet range, the visible range, and the infrared range. Since an absorption cross-section of particles typically peaks at different wavelengths for particles of different dimensions, a broad emission spectrum can be desirable to ignite the particles having different dimensions. A light-emitting diode that emits a short duration pulse also can be used as an optical source. Other embodiments can be implemented with an optical source that provides a substantially continuous light exposure to injected particles within a combustion chamber.

Examples of applications of the optical ignition technique described herein include the incorporation of optical ignition systems in combustion engines, such as those found in cars, power plants, aircrafts, and rockets; power generators in MEMS, such as for the purposes of sensors or actuators; and reaction devices, such as explosive devices for the purposes of demolition or weaponry. For example, rapid reciprocating engines and jet engines, such as air breathing jet engines, gas turbines, turbojet engines, turbofan engines, turboprop engines, prop fan engines, ramjet engines, and scramjet engines, can benefit from the incorporation of optical ignition systems to reduce the time for combustion and achieve higher combustion efficiency. For certain applications, a total combustion time can be specified as the time duration to burn x % of an initial mass of a fuel injected into a combustion chamber, as measured from the completion of delivery of a pulse of optical energy, where x % can be specified as 90%, 95%, 98%, 99%, 99.5%, or 100%, and where the mass of the fuel can be measured by, for example, a pressure trace from an initial pressure (e.g., in the range of about 1 μm to about 4 μm) using a piezoelectric pressure transducer. In some embodiments, a total combustion time of a distributed, optical ignition system can be significantly shorter relative to the use of conventional, single-point ignition systems, and can be no greater than about 150 ms, such as no greater than about 120 ms, no greater than about 100 ms, no greater than about 80 ms, no greater than about 60 ms, no greater than about 40 ms, or no greater than about 20 ms, and down to about 10 ms, down to about 5 ms, down to about 1 ms, or less. Optical ignition systems can provide additional advantages, such as lower cost and tower input power than conventional spark igniters. In addition, the optical ignition systems can be readily installed in a variety of combustion chambers, and can provide distributed ignition of fuels without requiring extreme high pressure or temperature.

Attention turns to FIG. 1, which illustrates a reciprocating engine 100 implemented in accordance with an embodiment of this disclosure. The engine 100 includes a housing 102, which defines a combustion chamber 104, and a piston 106, which is disposed within and is slidingly engaged with the housing 102 to allow up and down, reciprocating movement of the piston 106. As illustrated in FIG. 1, a set of injection mechanisms 108 and 110 are connected to and extend through the housing 102 to introduce a fuel and a set of particles (either, or both, nanoparticles and microparticles) into the combustion chamber 104, and an exhaust mechanism 112 is connected to and extend through the housing 102 to remove combustion gases or other products from the combustion chamber 104. The injection mechanisms 108 and 110 and the exhaust mechanism 112 can be implemented using, for example, valves, camshafts, nozzles, or a combination thereof Although the separate injection mechanisms 108 and 110 are illustrated in FIG. 1 to respectively introduce the fuel and the particles, it is contemplated that the fuel and the particles can be pre-mixed, and can be introduced into the combustion chamber 104 through a common injection mechanism.

In the illustrated embodiment, the engine 100 includes an optical ignition system, which includes an optical source 114 and a controller 116, which is connected to the optical source 114 and coordinates operation of the optical source 114 relative to the other components of the engine 100. The controller 116 can be implemented in hardware, software, or a combination of hardware and software. Upon injection of the fuel and the particles into the combustion chamber 104, the controller 116 activates the optical source 114, which irradiates the particles through an optical window 118 included in, or otherwise connected to, the housing 102 and, thereby, ignites the fuel in a distributed fashion. Resulting combustion products expand and push the piston 106 downwardly, and removal of the combustion products from the combustion chamber 104 allow upward movement of the piston 106 back to its initial position. Various aspects of the optical ignition system illustrated in FIG. 1 can be implemented as explained in the introductory passages above, and, therefore, are not repeated. It is contemplated that an internal surface of the housing 102 defining the combustion chamber 104 can be implemented as a reflective surface to enhance efficiency of optical ignition through scattering or other effects.

Attention next turns to FIG. 2, which illustrates a jet engine 200 implemented in accordance with another embodiment of this disclosure. The engine 200 includes a housing 202, which defines three sections, namely an inlet 204 to compress incoming air, a combustion chamber (or combustor) 206 to inject a fuel and a set of particles (either, or both, nanoparticles and microparticles) and combust the fuel, and a nozzle 208 to expel combustion products and produce thrust. An inlet body 210 is disposed within the housing 202 and at least partially extends through the inlet 204 to aid in the compression and to direct the flow of incoming air. As illustrated in FIG. 2, a set of injection mechanisms 212 and 214 are connected to and extend through the housing 202 to introduce the fuel and the particles into the combustion chamber 206. Although the separate injection mechanisms 212 and 214 are illustrated in FIG. 2 to respectively introduce the fuel and the particles, it is contemplated that the fuel and the particles can be pre-mixed, and can be introduced into the combustion chamber 206 through a common injection mechanism.

In the illustrated embodiment, the engine 200 includes an optical ignition system, which includes an optical source 216 and a controller 218, which is connected to the optical source 216 and coordinates operation of the optical source 216 relative to the other components of the engine 200. Upon injection of the fuel and the particles into the combustion chamber 206, the controller 218 activates the optical source 216, which irradiates the particles through an optical window 220 included in, or otherwise connected to, the housing 202 and, thereby, ignites the fuel in a distributed fashion. Resulting combustion products are expelled through the nozzle 208, thereby producing thrust. Various aspects of the engine 200 and the optical ignition system illustrated in FIG. 2 can be implemented as explained in the introductory passages and in connection with FIG. 1 above, and, therefore, are not repeated. It is contemplated that an internal surface of the housing 202 defining the combustion chamber 206 can be implemented as a reflective surface to enhance efficiency of optical ignition through scattering or other effects.

FIG. 3 illustrates a reaction device 300 that is implemented in accordance with yet another embodiment of this disclosure. The device 300 is implemented as an explosive device, and includes a housing 302, which defines an internal chamber 304 within which a reaction material 306 is disposed. In the illustrated embodiment, the reaction material 306 is implemented in a pellet form, and includes a substantially optically transparent fuel 308 and a set of particles 310 (either, or both, nanoparticles and microparticles) dispersed within the fuel 308. Also disposed within the internal chamber 304 is an optical ignition system, which includes an optical source 312 and a controller 314, which is connected to the optical source 312 and coordinates operation of the optical source 312 to ignite the fuel 308 and trigger detonation of the device 300. At a specified or user-selectable time, the controller 314 activates the optical source 312, which irradiates the particles 310 dispersed within the fuel 308, thereby igniting the fuel 308 in a distributed fashion. Various aspects of the device 300 and the optical ignition system illustrated in FIG. 3 can be implemented as explained in the introductory passages and in connection with FIG. 1 and FIG. 2 above, and, therefore, are not repeated. It is contemplated that an internal surface of the housing 302 defining the internal chamber 304 can be implemented as a reflective surface to enhance efficiency of optical ignition through scattering or other effects. It is also contemplated that the optical ignition system can be disposed outside of the housing 302, and can irradiate the reaction material 306 through an opening or optical window included in, or otherwise connected to, the housing 302.

EXAMPLE

The following example describes specific aspects of some embodiments of this disclosure to illustrate and provide a description for those of ordinary skill in the art. The example should not be construed as limiting this disclosure, as the example merely provides specific methodology useful in understanding and practicing some embodiments of this disclosure.

Aluminum, due to its large volumetric energy density of about 83.8 kJ/cm³, is an important fuel for solid rocket propulsion, high temperature processing, and MEMS. For example, small amounts of Al are integrated into MEMS to generate heat, microthrusts, and gases for actuation and power supply. However, a reliable and optical ignition technique is desirable for practical utilization of Al fuel. Especially for MEMS applications, the small Al quantity and feature size impose a challenge for reliable ignition with common ignition methods requiring physical contact, such as hotwires, heaters, and piezoelectronic igniters. Optical ignition by flash, instead, is very attractive because it works without physical contact and can achieve distributed ignition at multiple locations, thereby increasing reliability of ignition and flexibility of design. Optical flashes can be used to ignite Al nanoparticles (NPs) and other nanostructures, including carbon nanotubes, silicon nanowires, and graphene oxide. In these cases, the flash heats up the nanostructures to temperatures beyond their ignition temperatures by the photothermal effect, leading to ignition. Compared to Al NPs, Al microparticles (MPs) can be more suitable for practical systems since they are cheaper, safer to handle, and contain much higher Al content due to the smaller fraction of dead volume and weight of an inert Al₂O₃ shell. Nevertheless, it can be difficult to ignite Al MPs by flash due to their low light absorption and high ignition temperature, and ignition typically occurs with a large flash energy (e.g., >1 J/cm² from a xenon flash lamp).

This example investigates the effect of adding WO₃ NPs on the flash ignition of Al MPs. It is observed that the minimum flash ignition energy of Al MPs is greatly reduced by adding WO₃ NPs because WO₃ NPs improve both oxygen supply and light absorption from the flash. The combustion of Al MPs also can occur faster with the addition of nanoscale metal oxides.

Mixtures of Al MPs and WO₃ NPs are prepared by ultra-sonication using dimethylformamide (DMF) as a solvent. To study the efficacy of WO₃ NP-assisted flash ignition on Al MPs of different sizes, two different sizes of Al MPs, namely about 2.3 μm (Atlantic Equipment Engineers) and about 0.9 μm (Sigma-Aldrich) are separately mixed with WO₃ NPs. Al MPs and WO₃ NPs (about 80 nm, SkySpring Nanomaterials) are each weighed to satisfy the targeted fuel/oxidizer equivalence ratio while keeping a total mass of about 0.530 g. The Al MP and WO₃ NP mixture is added to about 10 ml of DMF and sonicated for about 15 min to ensure uniform mixing. After sonication, the mixture is gently dried on a hotplate at about 100° C. for about 6 h to remove the DMF. Finally, the dried mixture powder is passed through a 140 mesh (105 μm) sieve to break up large agglomerates. The Al-to-WO₃ equivalence ratio (φ) and normalized equivalence ratio (φ_(n)) of the mixture are represented in the following equation:

$\begin{matrix} {{\varphi = \frac{\left( \frac{m_{Al}}{m_{{WO}\; 3}} \right)}{\left( \frac{m_{Al}}{m_{{WO}\; 3}} \right)_{st}}},{\varphi_{n} = \frac{\varphi}{1 + \varphi}},} & (1) \end{matrix}$

where m_(Al) and M_(WO3) refer to the mass of Al and WO₃, respectively, and the subscript st refers to the stoichiometric condition for the reaction 2Al+WO₃→Al₂O₃+W. Although the Al MPs are encapsulated by a native inert Al₂O₃ shell (about 2-5 nm), the active Al content is about 97.5% for the small (about 0.9 μm) and about 99.0% for the large (about 2.3 μm) Al MPs with a 5 nm shell. Here, it is assumed that the entire mass of the Al MPs is Al when calculating the equivalence ratios.

A schematic of a flash ignition experimental setup is shown in FIG. 4( a). Flash ignition of the mixture of Al MPs and WO₃ NPs is achieved by a commercial camera flash (AlienBees™ B1600 Flash Unit) equipped with a xenon ring lamp with a maximum areal flash energy density up to about 0.84 J/cm² per flash. The areal energy density of the flash at each power setting is measured using an optical power detector (XLP12-35-H2, Gentec-EO USA, Inc). The mixture of Al MPs and WO₃ NPs is placed on top of a 1 mm thick glass slide that is placed directly above the xenon ring lamp. For each flash ignition experiment, the mixture powder is gently packed into a cylindrical shape (diameter: about 6.8 mm, height: about 2.6 mm) to maintain the same volume and cross-section area exposed to the flash (FIG. 4( b)). To measure the minimum flash ignition energy, the power of the flash is increased gradually until flash ignition occurs. Flash ignition experiments are carried out both in air and in N₂ atmospheres. For the flash ignition experiments in N₂, the entire flash unit and samples are placed inside an inflatable polyethylene glove box (Atmosbag glove bag, Sigma-Aldrich) that is purged substantially constantly with about 99.95% pure N₂.

The wavelength-dependent light absorption properties of various mixture samples are obtained with an integrating sphere using a xenon lamp coupled to a monochromator (Model QEX7, PV Measurements, Inc.). For the reflectance (R %) measurement, the samples are mounted at the backside of the integrating sphere, and the reflectance spectra are normalized to the reflection of a white-standard. The transmittance spectra (T %) are obtained by comparing the transmittance of test samples with a calibrated Si reference photodiode. Since the scattering component is not separately counted in the measurement, the absorption (A %) and scattering (S %) are calculated with the formula, (A+S)=100%−T %−R %.

Optical images in FIG. 4( b) show the sample appearance before (inset) and after the flash ignition in air, where the sample is a stoichiometric mixture of Al MPs (about 2.3 μm) and WO₃ NPs (about 80 nm) with φ=1. After ignition, the mixture color changes from gray to dark blue. The dark blue color indicates that some WO₃ (yellow) is reduced to WO_(3-x) (blue), and not completely to W (silver or gray color). The sample is spread out over a larger area after ignition, suggesting a violent reaction that is accompanied with gas expansion. Before flash ignition, Scanning Electron Microscopy (SEM, FEI XL30 Sirion, 5 kV) images (FIG. 5( a)) show that the spherical Al MPs (about 2.3 μm) and the WO₃ NPs are mixed, forming a densely packed powder. After the flash ignition, the products form much larger particles that are tens of microns in size (FIG. 5( b)), suggesting that melting and fusion occur together with reaction. These micron-sized particles are further analyzed by X-ray diffraction (XRD, PANalytical)(Pert 2, Cu Ka, 45 kV, 40 mA); the dominant peaks of the XRD pattern (FIG. 5( c)) are those of W and Al_(x)WO₃, indicating that WO₃ has been reduced, and some portion has formed aluminum-tungsten bronze. The absence of WO_(3-x) and Al₂O₃ peaks in the XRD pattern is likely because they are either amorphous or poorly crystalline. These characterizations confirm that reaction has occurred between Al MPs and WO₃ NPs upon exposure to a single optical flash. Flash ignition occurs when the energy of the incident light absorbed by the mixture of Al MPs/WO₃ NPs is sufficient to raise the mixture temperature beyond its ignition temperature.

WO₃ NPs influence the flash ignition of Al MPs in at least two ways: (i) increasing light absorption and (ii) decreasing ignition temperature by supplying oxygen to Al. To quantify the effect of WO₃ NPs, the minimum flash ignition energy (E_(min)) for the mixture of Al MPs and WO₃ NPs is plotted as a function of normalized Al/WO₃ equivalence ratio in both air (squares) and inert N₂ (circles) in FIG. 6. FIGS. 6( a) and 6(b) correspond to larger Al MPs (about 2.3 μm) and smaller Al MPs (about 0.9 μm), respectively. The error bars are established by performing three experiments with substantially identical conditions, and represent the range of measured E_(min) within the three measurements. First, both E_(min) curves (FIGS. 6( a) and 6(b)) show a concave shape, with higher E_(min) values in both the Al lean and rich regions. In this example, the lowest point of the E_(min) curve does not correspond to the stoichiometric condition of reaction 2Al+WO₃→Al₂O₃+W (φ=1, φ_(n)=0.5), since WO₃ is not completely reduced to tungsten in the flash experiment. Second, when Al is the deficient species with respect to WO₃ (φ_(n)<0.5), E_(min) in air is comparable to that in N₂. That E_(min) does not increase when gaseous O₂ is removed suggests that Al MPs are preferentially oxidized by the WO₃ NPs. WO₃ NPs may be more effective oxidizers than air because of their large contact area with the Al MPs, which facilitates ignition through the diffusion-based reactive sintering mechanism. Third, adding WO₃ NPs to pure Al MPs (φ_(n)=1) or to Al MPs in excess supply with respect to WO₃ (φ_(n)>0.5), i.e., changing φ_(n) from 1 to 0.5, significantly lowers E_(min) because WO₃ oxidizes Al more effectively than air. Finally, when comparing Al MPs of two different sizes, E_(min) for the 2.3 μm Al MPs is on average about 0.4 J/cm² higher than that for the 0.9 μm Al MPs, which is consistent with a higher ignition temperature of about 1,600 K for larger (about 2.3 μm) particles compared to about 1,400 K for smaller (about 0.9 μm) particles. Nevertheless, addition of WO₃ NPs reduces E_(min) for Al MPs of both sizes.

The above E_(min) measurements show that WO₃ NPs are more effective oxidizers than air for Al MPs. In addition, WO₃ NPs can also enhance the light absorption of the mixture of Al MPs/WO₃ NPs upon flash exposure, which can increase the temperature rise due to the photothermal effect. FIG. 7 shows the light absorption spectra of several mixtures of Al MPs and WO₃ NPs over wavelengths of 300-1100 nm. FIGS. 7( a) and 7(b) correspond to larger (about 2.3 μm) and smaller (about 0.9 μm) Al MPs, respectively. First, for pure Al MPs (φ_(n)=1.0), the slight absorption increase around 830 nm corresponds to the interband transition frequency of about 1.5 eV for aluminum. For the other samples containing WO₃ NPs, light absorption is increased for wavelengths below about 460 nm, consistent with the WO₃ bandgap of about 2.7 eV (about 460 nm). Second, the smaller Al MPs absorb about 15% more light than the larger Al MPs, facilitating the flash ignition process. Third, the total light absorption is calculated by integrating the product of the xenon flash spectrum and the absorption plus scattering spectrum over 300-1100 nm wavelengths. The total light absorption is increased by about 12.2% and about 1.4% for the larger (about 2.3 μm) and smaller (about 0.9 μm) Al MPs, respectively, when a stoichiometric quantity of WO₃ NPs (φ_(n)=0.5) is added to the pure Al MPs. Since the light absorption enhancement due to the addition of WO₃ NPs is negligible for the smaller Al MPs, the reduction in E_(min) is mainly attributed to effective oxygen supply by WO₃ NPs due to their intimate and large contact area with Al MPs. On the other hand, for larger Al MPs, the reduction of E_(min) by WO₃ addition results from both effective oxygen supply and enhanced light absorption.

By way of summary, this example presents a study of the effect of WO₃ NP addition on the flash ignition of Al MPs of two different sizes by measurement of the minimum flash ignition energy, E_(min). E_(min) is greatly reduced by the addition of WO₃ NPs for Al MPs of both sizes. For the smaller Al MPs, the reduction of E_(min) mainly derives from the more effective oxygen supply by WO₃ NPs than by air. For the larger Al MPs, the E_(min) reduction is due to the combined effects of effective oxygen supply and light absorption enhancement by WO₃ NPs. These results extend the flash ignition of more expensive and lower energy density Al NPs to inexpensive and higher energy density Al MPs.

While the invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention as defined by the appended claims. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, method, operation or operations, to the objective, spirit and scope of the invention. All such modifications are intended to be within the scope of the claims appended hereto. In particular, while certain methods may have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the invention. Accordingly, unless specifically indicated herein, the order and grouping of the operations is not a limitation of the invention. 

What is claimed is:
 1. A method of combustion, comprising: introducing microparticles and nanoparticles into a combustion chamber, wherein the microparticles and the nanoparticles are formed of different materials; and using an optical source, irradiating the microparticles and the nanoparticles within the combustion chamber to ignite the microparticles.
 2. The method of claim 1, wherein the nanoparticles are formed of a metal oxide.
 3. The method of claim 2, wherein the metal oxide is tungsten oxide.
 4. The method of claim 2, wherein the microparticles are formed of a metal.
 5. The method of claim 4, wherein the metal is aluminum.
 6. The method of claim 4, wherein a molar ratio of the metal oxide to the metal is in the range of 1:10 to 1:1.
 7. The method of claim 1, wherein irradiating the microparticles and the nanoparticles is carried out at an energy density up to 1 J/cm².
 8. The method of claim 7, wherein the microparticles have sizes in the range of 500 nm to 1 μm.
 9. The method of claim 7, wherein the microparticles have sizes in the range of 1 μm to 10 μm.
 10. A reaction device comprising: a housing defining an internal chamber; a reaction material disposed within the internal chamber and including microparticles and nanoparticles that are formed of different materials; and an optical ignition system connected to the housing and operable to irradiate the microparticles and the nanoparticles to ignite the reaction material.
 11. The reaction device of claim 10, wherein the microparticles are formed of a metal, and the nanoparticles are formed of a metal oxide.
 12. The reaction device of claim 11, wherein the metal is aluminum.
 13. The reaction device of claim 11, wherein the metal oxide is tungsten oxide.
 14. The reaction device of claim 11, wherein a molar ratio of the metal oxide to the metal is in the range of 1:10 to 1:1.
 15. The reaction device of claim 11, wherein a molar ratio of the metal oxide to the metal is in the range of 1:5 to 1:1.
 16. The reaction device of claim 11, wherein the optical ignition system is operable to irradiate the microparticles and the nanoparticles at an energy density up to 1 J/cm².
 17. The reaction device of claim 16, wherein the microparticles have sizes in the range of 500 nm to 1 μm.
 18. The reaction device of claim 16, wherein the microparticles have sizes in the range of 1 μm to 10 μm. 